Nov. 27, 1884] 



NA TURE 



87 



of its performance, of which the writer of these lines can 

 personally testify, is already widely acknowledged. 



The main peculiarity of the new instrument lies in the 

 fact that, whereas in almost all galvanometers there is a 

 fixed coil and a movable magnetic needle, in this gal- 

 vanometer the coil is movable and the magnet — no 

 longer a mere needle but a substantial compound horse- 

 shoe of steel — is fixed. Fig. 1 represents the instrument 

 itself. The steel magnet, made of three thin horse-shoes 

 each magnetised as strongly as possible, is firmly fixed 

 to a metal base, with its poles upwards. Between the 

 poles hangs the coil, rectangular in form and extremely 

 light, held in its place by a thin silver wire above and 

 another thin silver wire below. This coil is made by 

 winding on a rectangular core, which, after the strands 

 have been cemented and bound together, is removed, 

 leaving the wire only. It weighs only a few grains. 

 To reinforce the magnetic field a small cylinder of soft 

 iron, small enough to lie in the hollow of the suspended 

 rectangular coil without touching it, is placed between 

 the poles and is rigidly supported from behind. The coil 

 is then free to turn in the very narrow space between the 



iron core and the external magnet-poles ; and it need 

 hardly be added that this contrivance produces a very 

 intense magnetic field. The current is led in by one of 

 the silver suspension-wires, and leaves the coil by the 

 other. So far the arrangement precisely resembles that 

 adopted in the well-known "siphon-recorder" of Sir W. 

 Thomson, invented twenty years ago for the purpose of 

 cable-signalling. A small mirror of 1 metiefocus is affixed 

 to the suspended coil ; a brass spring at the bottom keeps 

 the suspending wires adequately stretched ; and a screw- 

 head at the top of the instrument serves both to regulate 

 the tension in the wires and to let the coil down, to a 

 position of rest on the central iron cylinder, whenever the 

 galvanometer is to be dismounted for removal to a distant 

 place. The resistance of the coil is about 150 ohms in 

 the ordinary pattern of instrument. As there is no sus- 

 pended needle, no external magnetic forces affect the zero 

 of the instrument ; and, since the position of the coil is 

 determined solely by the elasticity of the suspending wires 

 and the magnetic action of the fixed magnet on the current 

 in the coil, it can be used in any position, and is inde- 

 pendent of the earth's magnetic field. It can even be 



placed quite near to a dynamo-machine. The intensity 

 of the magnetic field in which the coil is situated is such 

 that whenever the galvanometer-circuit is closed — even 

 through a considerable resistance— the motion of the 

 needle is dead-beat. It takes less than one second to 

 come to rest at its final position of deflection, and when 

 it returns to zero it does so with the most complete 

 absence of oscillations. The spot of light on the scale 

 never oscillates so much as 1 millimetre over the zero 

 on releasing the galvanometer-key. 



The optical arrangements adopted by M. Carpentier 

 are shown in Fig. 2. The instrument is set with its three 

 levelling screws in three V-grooves in a convenient bed- 

 plate. Opposite it is set a semi-transparent scale of cellu- 

 loid, 50 centimetres in length, graduated in millimetres. 

 The light is provided by a single wax candle held in a 

 holder like a carriage-candle, which also carries a para- 

 boloidal mirror back. This candle is set so that its light 

 falls upon a small adjustable plane mirror fixed to the 

 back of the scale. This mirror reflects the ray upon the 

 small mirror of the galvanometer, but as it passes be- 

 neath the scale it traverses a square aperture across 

 which a thin wire is stretched vertically. To see the 

 spot of light the observer stations himself in front of 

 the scale, so as to see the light coming through the strip 



of celluloid. He sees a bright patch about 1 inch square 

 having a single sharply-defined black line — the image of the 

 aforesaid wire — down its middle. This patch of light and 

 the central line are perfectly visible in broad daylight, but 

 cannot be well seen by more than one observer at one 

 time. The adjustment of the lamp and scale is a simple 

 matter ; and light from any lamp in the room — an overhead 

 gas-light for example — may be used instead by turning the 

 adjustable mirror to the proper angle. 



When set up without any shunt, this galvanometer will 

 show a deflection of 1 millimetre on the scale for about 

 1/100,000,000 of an ampere of current : but the sensitive- 

 ness differs in different instruments with the construction 

 of the coil, the stiffness of the suspension, and the power 

 of the magnets. Two instances of its application may be 

 given. 



The instrument can be applied as a volt-meter to measure 

 the electromotive forces of cells in the manner indicated 

 in Fig. 3. An ordinary reversing-key, k, is connected to the 

 galvanometer, and an adjustable resistance (a Wheatstone 

 rheostat with a thin wire having a range from 1 to 200 

 ohms is convenient) is interposed as a shunt, s. To cali- 

 brate the instrument a standard Daniell cell (E.M.F. 

 = 1 -07 volt) is placed at B in circuit with a resistance box. 

 A resistance of 10,000 ohms is unplugged and a reading 



